The invention relates, in general, to control systems for prime movers and, in particular, to an electronic control system for an extraction type steam turbine.
The invention is applicable to cogeneration type power plants which output both power and process steam. This type of power plant may include several turbine-generator sets and at least one extraction steam turbine. U.S. Pat. No. 2,977,768 to Wagner and Straney issued Apr. 4, 1961 sets forth a basic electronic control system for an extraction type steam turbine. The patent describes two signal paths; i.e., a speed signal path and a pressure signal path which ultimately set the position of the turbine inlet valve and the extraction valve. The electronic signal paths are interconnected because the speed error signal modifies the pressure error signal and the pressure error signal modifies the speed error signal. This leads to the following result. If a change in turbine load causes a speed error, corrective action is taken by the inlet steam valve and the extraction steam valve moving in the same direction in order to maintain extraction pressure and flow constant. If on the other hand, a change in process steam requirement generates an extraction pressure error signal, the inlet valve and the extraction valve move in opposite directions to maintain constant speed and correct for the pressure error.
U.S. Pat. No. 3,391,539 to Dimitroff and Wagner describes a control system for a multiple turbine power plant which allocates and dispatches the flow of process steam in a pre-determined order of preference. The present invention differs from the Dimitroff et al patented invention in that process steam availability is subject to load requirements based upon inlet steam flow/pressure. The invention is directed to a control system which will automatically switch from a speed/extraction pressure control mode to an inlet header pressure/speed control mode in order to ensure that available inlet steam is used to satisfy the preferred load requirements rather than process steam requirements should the power plant inlet steam flow fall short of normal plant cogeneration requirements. The latter short fall could occur if a boiler were temporarily taken out of service.
U.S. Pat. No. 3,971,219 to Taylor and Finck shows a control system for an extraction type steam turbine wherein the turbine inlet valve is positioned to achieve maximum boiler performance and wherein the extraction valve maintains speed control for the turbine in accordance with the position of the inlet valve position. Extraction flow may be achieved by a supplemental boiler. The present invention differs in circumstance and control mechanism in that more than one steam demand is being considered and extraction pressure is controlled when there is available steam.